even if it's an OS or kill bug, most processes do reliably get killed with -9 so it's something that this program is doing is where to look/how to reproduce.
It shouldn't matter what the program is doing or not, it's an OS (or kill) level bug. Even if you can make changes to the program so it doesn't happen, it's still an OS/kill level bug. The OS is supposed to terminate the process. A process is not supposed to be able to prevent the OS from doing that.
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even if it's an OS or kill bug, most processes do reliably get killed with -9 so it's something that this program is doing is where to look/how to reproduce.
It shouldn't matter what the program is doing or not, it's an OS (or kill) level bug. Even if you can make changes to the program so it doesn't happen, it's still an OS/kill level bug. The OS is supposed to terminate the process. A process is not supposed to be able to prevent the OS from doing that.
correct, and virtually all programs get killed. so what is this program doing that seems to uncover the bug?
"shouldn't" hides the flaw in your thinking, it is happening, that's the bug, so shouldn't is out the window